It has been proposed that telomere repeat sequences and telomere terminal transferase (telomerase) may influence mutation induction by ionizing radiation. Interstitial telomere repeat sequences are thought to be radiation-sensitive fragile sites, and telomerase has been reported to cap the ends of broken chromosomes, preventing their repair and promoting chromosome terminalization or recombination. We have been characterizing a Chinese hamster ovary cell line into which a gpt containing retroviral shuttle vector has been stably integrated. This normally stable locus in T5 cells is very sensitive to deletion mutation following ionizing radiation exposure. The gene also shows an LET response with an RBE of 3 for a particles. Analysis of the integration site has identified regions of T2AG3 telomere repeats both 3-prime and 5-prime to the gpt gene. The goal of these studies is to establish the role of telomere repeat sequences and telomerase in radiation sensitivity and genomic stability using this gpt locus as a model system. We plan to test the following hypotheses: (l) the telomere repeat sequences found at the gpt integration site were added to the vector or to the vector integration site by telomerase prior to or during integration, (2) these telomeric sequences have made the vector integration site a radiation-sensitive site, and (3) telomeres act as radiation-sensitive fragile sites by either serving as a site for further telomerase action and chromosome terminalization or by providing repeat structures to facilitate radiation-induced recombination. These studies should further our understanding of the roles that telomerase and interstitial telomere sequences play in genetic disease and help to better define the potential risks associated with environmental exposures to both low- and high-LET radiations.